This invention relates to correcting non-uniformity of size or ejection velocity of each individual droplet of acoustic inkjet printheads and more particularly to an acoustic inkjet printhead where the RF signal delivered to each transducer is individually controlled to provide a desired droplet size or ejection velocity.
Referring to FIG. 1, there is shown a portion of a prior art acoustic inkjet printhead 10. Printhead 10 has a liquid level control structure 12 which captures an ink layer 16 over a glass substrate 14. Liquid level control structure 12 has a plurality of apertures 18 each of which is dedicated to a single acoustic ejector. Under the glass substrate, there is a plurality of piezo-electric transducers 20. For the purpose of simplicity, hereinafter, the "piezo-electric transducer" is referred to as "transducer." Each transducer 20 is dedicated to one aperture 18 and is located directly across its respective aperture 18. Once each transducer 20 is activated, it will oscillate and generate acoustic waves 22 which travel within the glass substrate 14 toward the ink 16.
Over the glass substrate 14, there is a plurality of Fresnel lenses 24, each of which corresponds to one of the transducers 20 and is located across from its respective transducer 20. The Fresnel lenses 24 receive the acoustic waves 22 from the transducers 20 and focus the acoustic waves onto the liquid surface in their respective apertures 18. The focused waves 22 cause the ink to be ejected from the apertures as droplets 26.
Oscillator 30 generates a RF signal and sends it to a RF Amplifier 32 to be amplified. The amplified RF signal is sent to several RF power switches 34. Each output of each power switch 34 is distributed between the plurality of transducers 20 where each transducer 20 is grounded through its individual switch Z. Switches Z control the time and duration of the RF signal applied to each transducer 20. Once each switch Z is closed, the RF signal flows into the respective transducer 20 and once the switch is opened, the flow of the RF signal into the transducer 20 will be significantly reduced. The time each switch Z is closed determines the duration of the RF signal applied to the transducers. All switches Z receive the same control signal C from a switch driver circuit 36. As a result, all switches Z, that are supposed to close, close at the same time and all the closed switches Z open at the same time. The electrical connection shown in FIG. 1 is designed to deliver a RF signal with the same power to each transducer 20. However, due to the variations in the focusing characteristics of different transducers 20, droplets 26 ejected from different apertures 18 might have different sizes.
Non-uniformity of droplet size 18 creates different size pixels which can be observed as variation of the thickness of a line. However, non-uniformity of ejection velocity causes pixels not to align on a straight line. Since the paper is moving, in order to have a straight line, all the droplets have to be ejected at the same time to strike the paper at the same time. If a droplet is ejected before or after the intended time, the pixel on the paper will be above or below the intended line respectively. Non uniformity of the droplet size and velocity are two problems of the prior art acoustic ink jet printheads.
It is an object of this invention to provide a driver circuitry to control the size or ejection velocity of droplets of acoustic inkjet printheads.